8 Jul 2005 AST 2010: Chapter 5 1

Astronomical Astronomical InstrumentsInstruments

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Early TelescopesEarly TelescopesAncient cultures built special sites, called observatories, for observing the skyAt these observatories, they were able to measure the positions of celestial objects that were visible to the naked eyeTelescopes were first used to observe the sky by Galileo Galilei, and so they are a relatively recent addition to the tools astronomers use

The use of telescopes, however, completely revolutionized our ideas about the universe

8 Jul 2005 AST 2010: Chapter 5 3

Galileo’s TelescopesGalileo’s TelescopesGalileo first used a telescope to observe the sky in 1610His telescopes were simple tubes held by hand

They were also small in comparison to the telescopes in use today

The use of these small telescopes allowed Galileo to revolutionize the field of astronomy

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Why Use Telescopes to Why Use Telescopes to Observe the Sky?Observe the Sky?

Celestial objects — planets, stars, galaxies, etc. — emit (or reflect) light in many different directionsOnly a minuscule fraction of the light emitted (or reflected) by celestial objects is captured by the human eye, with its tiny opening 

The light not shining into the eye is “wasted” Most objects of interest to astronomers are extremely faint

The more light from such objects we can collect, the better we can study them

A telescope is a very important tool because ithas a much larger opening than the human eye and, therefore, captures much more lightfocuses all the light collected into an image much better than the naked eye can

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Telescopes of All KindsTelescopes of All KindsStars and other celestial objects emit all types of electromagnetic waves, not only visible lightNowadays, there are types of telescopes that collect not visible light, but other forms of EM radiation, such as radio waves, infrared, ultraviolet, X-rays, and even gamma raysSuch telescopes may use collecting devices that look very different from the lenses and mirrors used in visible-light telescopes, but those devices serve the same function

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ApertureApertureIn telescopes of all types, the light-gathering ability is determined by the area of the device acting as the main collector of light (or other forms of electromagnetic radiation)The aperture of a telescope with round lenses and mirrors corresponds to the diameter of its primary lens or primary reflector (mirror) The light-gathering power of a telescope is determined by its apertureThe amount of light a telescope can collect increases with the square of the aperture

For example, an aperture with a 4-meter diameter can collect 16 times as much light as an aperture with a 1-meter diameter

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Need for ImagesNeed for ImagesThe study of astronomical objects requires the formation of their imagesOnce formed, each of the images can be

looked at directly with the naked eyeimprinted on a photographic filmdetected and recorded with various light-sensitive devices

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Telescope Images in HistoryTelescope Images in HistoryBefore the 20th century, telescope images were simply looked at with the naked eye

This was a rather inefficient and unreliable way of gathering/collecting and preserving the information

In the 20th century, before the arrival of computers, images were imprinted/recorded on photographic filmsNowadays, astronomers actually rarely look through the larger telescopes

Most images are recorded electronically on computers

Formation of Image by LensA convex lens is a transparent piece of material that bends parallel rays of light passing through it and brings them to a focus or focal point

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EyepieceEyepieceTelescopes use a combination of lenses and mirrors to produce imagesAn image formed by the primary lens of a telescope can be viewed, and magnified, by using a second, smaller lens called an eyepiece  Nowadays, the eyepiece of a telescope is usually replaced by a camera or electronic light detector

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MagnificationMagnificationThe eyepiece can magnify the imageStars are typically so distant that they appear as points of light, and consequently magnification does not do muchPlanets, however, are much closer, and galaxies much bigger, than stars so that magnification is actually quite useful to see the shape and structure of planets and galaxies

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Concave MirrorsConcave MirrorsA telescope can also be built using a concave mirror to form an image

Such a mirror reflects incoming parallel rays through its focus

Thus images can be produced by a concave mirror exactly as they are by a convex lens

Convex lensConvex lens

Concave mirrorConcave mirror

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Basic Designs of Visible-Light Basic Designs of Visible-Light TelescopesTelescopes

Refracting telescopes Reflecting telescopes

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Refracting TelescopesRefracting TelescopesIn a refractor (refracting telescope), the primary light-gathering device is a convex lens Galileo's telescopes were all refractors, as are today’s binoculars and opera glasses Refractors are not good for most astronomical applications

It is very difficult to make a large lens without flaws and support it without causing it to become distorted

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Reflecting Reflecting TelescopesTelescopes

Telescopes designed with mirrors avoid the problems of refractors with large lensesThe first successful reflecting telescope (reflector ) was built by Newton in 1668A concave mirror is placed at the bottom of the reflecting telescopeThe mirror reflects the light back up the tube to form an image near the front end at a location called the prime focusImages can be observed directly at the prime focus, or additional mirrors can be used to redirect the light to a more convenient location

Newton’s telescope

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Focus Arrangements for Focus Arrangements for Reflecting TelescopesReflecting Telescopes

Different options for where the light is brought to a focus

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Some Large-Aperture Some Large-Aperture Telescopes (1)Telescopes (1)

The Hale telescope on Palomar Mountain in southern California

is a reflectorwas built in 1948 has a mirror that is 5 meters (200 in) in diameterwas the world’s largest visible-light telescope for 45 years

Website

KeckKeck

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Some Large-Aperture Some Large-Aperture Telescopes (2)Telescopes (2)

Two Keck telescopes on (dormant) Mauna Kea volcano in Hawaii

became operational in 1993-1996each have a mirror that is 10 meters in diameter and composed of 36 hexagonal sectionsare sensitive to both visible and infrared wavelengths

Website

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Resolution of TelescopeResolution of TelescopeIn addition to collecting as much light as they can, astronomers also seek to get the sharpest images possible

Sharper images provide more details about the objects observed

The resolution of a telescope refers to the fineness of detail present in the images it producesThe resolution of an image is measured in units of angle on the sky

The angular size is typically expressed in arcsecondsOne arcsec is 1/3600 degree — a very tiny angle!One arcsec is how a quarter would look like when seen from a distance of 5 km (3 mi)

One of the factors that determine resolution is the telescope’s aperture

Larger apertures result in sharper images

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Important External Factor Important External Factor Limiting ResolutionLimiting Resolution

External factors, however, can also affect the resolution Turbulence in the Earth's atmosphere above a telescope results in the blurring of the images it produces

The “twinkling” of stars as seen with the naked eye from Earth is a result of atmospheric turbulenceIn the absence of the atmosphere, the light of stars appears steady

Therefore, it is important to place telescopes at high altitudes where atmospheric blurring is minimizedTelescopes mounted in outer space, above the Earth's atmosphere, are not affected by atmospheric blurring

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Adaptive OpticsAdaptive OpticsThe technique of adaptive optics can make corrections for atmospheric blurring

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Additional Factors Affecting Additional Factors Affecting Performance of TelescopesPerformance of Telescopes

The weather — clouds, wind, rain, etc — is the most obvious limitation

At the best sites, it is clear as much as 75% of the time

The atmosphere filters out certain amount of starlightWater vapor absorbs much of the infraredThe preferred sites are dry, at high altitudes

“Light pollution” in the sky often occurs near citiesThis is the scattering by air of the glare from city lights producing an illumination that hides the faintest starsThe best sites have dark sky, far away from large cities

The air is often unsteadyLight passing turbulent air is disturbed, resulting in blurred images — an effect called “bad seeing”

Sites with steady atmosphere are preferred

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Visible-Light Detecting (1)Visible-Light Detecting (1)After capturing the radiation from celestial objects, astronomers sort it according to wavelength

The instruments used may be as simple as colored filters or a complicated spectrometer

A spectrometer is an instrument designed to disperse light into a spectrum to be recorded for detailed analysis

Spectroscopy is one of the most powerful techniques used in astronomy

After the radiation passes through the sorting instruments, its properties are recorded and measured using detectors

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A Prism SpectometerA Prism Spectometer

Nowadays, the prism is replaced by a diffraction grating, which is a piece of transparent material with thousands of grooves in its surface that also cause the light waves to spread out into a spectrum

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Visible-Light Detecting (2)Visible-Light Detecting (2)Throughout most of the 20th century, photographic films, or plates, served as the main astronomical detectors

In a plate, a light-sensitive chemical coating is applied to a piece of glass which, when developed, provides a lasting record of the imageAlthough photographic films represent a large improvement over the human eye, they are inefficient, only recording about 1% of the light incident on them

Astronomers now use much more efficient electronic detectors to record images

Most often, these are charge-coupled devices (CCDs), which are similar to the detectors used in video camcorders or in digital camerasCCDs record up to 70% of the photons that strike them, resulting in much sharper images, and also provide output that can go directly to a computer for analysis

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Infrared ObservationsInfrared ObservationsObserving the sky in the infrared band presents additional challengesThe infrared extends from wavelengths near 1 micrometer out to 100 micrometer or longerInfrared radiation is basically heat radiation

The human body emits heat in the infrared rangeA big challenge: at typical temperatures on Earth’s surface, the telescope being used and the atmosphere are all emitting infrared radiation!

To infrared “eyes”, everything on Earth is brightly aglowThe challenge is to detect faint cosmic sources against this sea of infrared

The solution is to isolate the detector in very cold surroundings, often held near absolute zero temperature (1-3 kelvin), by immersing them in liquid helium

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Radio Telescopes Radio Telescopes Radio emission was discovered by Karl G. Jansky, an engineer of the Bell Telephone Laboratories, in 1931In 1936, Grote Reber built from galvanized iron and wood the first antenna specifically designed receive cosmic radio waves

Over the years, he built several antennas and conducted pioneering surveys of the sky for celestial radio sources

In commercial radio broadcasting, sound information is encoded at the source and decoded at the receiving ends, the listener's radios where they are played into headphones or speakersRadio waves from space do not contain music or other types of human informationThe waves nonetheless carry some information about the chemistry and physical conditions of their sources

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Radio AstronomyRadio AstronomyRadio waves can produce an electric current in conductors (such as metals)An antenna is such a conductor

It intercepts the path of waves which in turn induce a small current in itThe current is then amplified in a radio receiver and recorded

Receivers, like our TV or radio sets, can be tuned to select a single frequency (channel)An astronomical radio telescope provides radio spectra, giving information about how much radiation we receive at each wavelength or frequency

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Radio TelescopesRadio TelescopesA radio-reflecting telescope consists of a concave metal reflector, called a dish, quite analogous to an optical telescope mirrorThe radio waves collected by the dish are reflected to the focal point of the reflector where a receiver detects the waves and record them  Astronomers often construct a pictorial representation of the radio sources they observe in order to communicate and visualize their data more easily

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A Radio ImageA Radio Image

Colors have been added to help the eye sort out regions of different Colors have been added to help the eye sort out regions of different intensities. Red regions are the most intense, blue the leastintensities. Red regions are the most intense, blue the least

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Green Bank TelescopeGreen Bank TelescopeThis is the world's largest fully steerable radio telescope

located at the National Radio Astronomy Observatory in West Virginia with a dish about 100 meters acrossthat can be pointed to any direction in the sky

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Arecibo Telescope (1)Arecibo Telescope (1)It is the world’s largest radar telescope

consisting of a 305-m fixed reflecting surface, made up of 40,000 individual panelsIt is suspended in a natural limestone sinkhole in northwestern Puerto Rico

Incoming rays are reflected back from the surface to two additional reflectors located 450 feet above on the “platform”, a 500-ton structure supported by cables from three towers

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Arecibo Telescope (2)Arecibo Telescope (2)

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Radio Interferometry (1)Radio Interferometry (1)A telescope resolution depends primarily on its apertureIt also depends on the wavelength of the wave being detected

The longer the waves, the harder it becomes to detect fine details

Radio waves have very large wavelengthsSubstantial challenges for astronomers who need good resolution

The largest radio dishes cannot have poorer resolution than small optical telescopesTo overcome this difficulty, astronomers have learned to link two or more radio telescopes together electronically, and succeeded in greatly sharpening the images they get

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Radio Interferometry (2)Radio Interferometry (2)An array of telescopes linked together in this way is called an interferometerThe word indicates that these devices operate via a measurement of the degree of interference between different waves

Interference is a technical term for the way waves that arrive in a detector at slightly different times interact with each other

The resolution of an interferometer depends on the separation of the telescopes, not on their individual aperturesEven better resolution can be achieved by combining more than two reflectors into an interferometer array

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VLAVLAThe most extensive such instrument is the National Radio Astronomical Observatory's Very Large Array (VLA) near Socorro, New Mexico

It consists of 27 movable radio telescopes, each having an aperture of 25 m, spread over a total span of about 36 kmThe telescopes signals are combined electronically and permit astronomers to obtain pictures of the sky with a resolution comparable to those obtained with an optical telescope with a resolution of about 1 arcsecond

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Observations Outside Earth's Observations Outside Earth's Atmosphere Atmosphere

Earth's atmosphere blocks most radiation at wavelengths shorter than those of visible light

It is thus possible to make astronomical observation at these wavelengths only from space

Getting around the disturbing effects of the atmosphere is also a great advantage at visible and infrared wavelengthsSince stars do not twinkle in empty space, the resolution is far superior than that on Earth The resolution thus becomes solely limited by the size and quality of the instrument used to collect the light

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Hubble Space Telescope (1) Hubble Space Telescope (1) Launched in April 1990, it permitted a giant leap forward in astronomyIt has an aperture of 2.4 m

the largest of those put in space to datelimited by the payload of the space shuttle used to put it in orbit

It was named after Edwin Hubble, the astronomer who discovered the expansion of the universe in the 1920s

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Hubble Space Telescope (2)Hubble Space Telescope (2)It is operated jointly by NASA’s Goddard Space-Flight Center and the Space Telescope Science Institute in BaltimoreIt was the first orbiting observatory designed to be serviced by shuttle astronautsVisits by astronauts in 1993, 1997, and 1999 allowed improvements and replacements of the initial instruments